In the prior art (e.g., WO 01/55602 A1) hydropneumatic pressure accumulators are known, with a bellows separating a gas space from an oil space within the pressure accumulator housing. The bellows, especially in the form of a metal bellows, is attached on its one end to the pressure accumulator housing, such that the oil space borders the inside of the bellows. On its other free end, the bellows is sealed by a closure body which can be moved according to volume changes of the gas space and oil space as the two working chambers of the pressure accumulator. A valve blocks or releases the flow of the hydraulic fluid out of and into the oil space. The movement of a closure body corresponding to an increase in the volume of the gas space exceeding a given maximum value moves the closure body into its blocking position. The closure body is made in the form of a trough with a bottom designed as a movable valve member of the valve to control the flow of the hydraulic fluid medium.
As is recognized, in bellows-type pressure accumulators with rubber bellows or metal bellows, care must be taken that overloading of the bellows be avoided. In another known pressure accumulator (WO 97/46823 A1), with respect to this problem a valve stem of the valve connected to the oil space is configured relative to the closure body of the metal bellows in a positional relationship such that the closure body of the metal bellows made as a flat end plate acts on the valve stem when a desired end position is reached and moves it into the blocking position of the valve. The outflow of the hydraulic fluid from the oil space is then stopped when this end position of the end plate of the metal bellows is reached. With the valve closed, even when the connected hydraulic system should become unpressurized, a pressure is maintained in the oil space of the pressure accumulator corresponding to the gas pressure prevailing at the time in the gas space so that a pressure equilibrium prevails on the metal bellows on either side.
This arrangement prevents overloading of the bellows when in operation of the pressure accumulator the pressure of the hydraulic system connected on the oil side decreases. However, there is the danger of damage to the bellows in states with an overpressure prevailing on the oil side or in the absence of the prefill pressure on the gas side. Since in this known pressure accumulator the maximum value of the volume of the gas space corresponds essentially to the stroke volume defined by the motion of the end plate taking place when the metal bellows contracts and expands, the stroke length which the end plate can traverse within the pressure accumulator housing must be selected to be relatively long if a gas space volume sufficient for pressure accumulator operation is to be made available. In the absence of a gas prefill pressure or overpressure prevailing on the oil side, the prevailing pressure gradient acts on the fully extended and thus mechanically most heavily loaded metal bellows. It is therefore necessary to use either thicker or multilayer metal bellows. Disadvantageously for this reason the spring stiffness is greatly increased. This increased spring stiffness leads to a comparatively poor response behavior in operation. Multilayer bellows lead to increased weight and higher costs. Moreover the stroke per turn of the bellows is less.
In WO 01/55602 A1, a valve stem is attached to the trough bottom, extends concentrically to the longitudinal axis out of the pressure accumulator housing, and is connected to a second movable valve element. When the motion of the trough exceeds a given minimum value of the volume of the gas space, the valve stem interacts with a second valve seat to block the flow of the hydraulic fluid into the oil space. The advantageous possibility then arises of controlling the end position of the trough corresponding to the minimum value of the volume of the gas space using an oil-side valve. Since in the known solution the entire interior of the trough is available as part of the gas space, an optimum ratio is achieved between the total size of the pressure accumulator housing and the volume of the gas space, although the volume to be assigned to the gas space for accommodation and management, especially in the form of pulsation damping for the hydraulic fluid as a further fluid, cannot then be available. In the known solution, the pressure accumulator housing can be shaped such that it forms a mechanical stop after short stroke motion of the trough, because the entire interior of the trough is available as a gas space volume. The metal bellows as a whole is protected not only against overly strong expansion, but since it surrounds the outside of the indicated trough, the bellows at the overpressure prevailing in the gas space is also mechanically supported on the outside of the trough over the entire length. In spite of this circumstance and in spite of the existing very small “dead volume” between the trough and bellows, it however cannot be precluded that individual folds of the metal bellows are still unduly exposed to stresses and can tear and fail. Furthermore, both in the area of the valve element and in the area of the possible trigger point between the trough which can move lengthwise and the inside wall of the pressure accumulator housing, seals are necessary which are fundamentally subject to wear and consequently can lead to failure of the known hydropneumatic pressure accumulator solution.